This invention relates to detectors for ionizing radiation, such as x-ray and gamma radiation. The invention is concerned with improving multicell detectors by minimizing thermal and microphonic instabilities of the detectors.
The detectors have various uses but are especially useful in x-ray computed axial tomography systems. In the computed axial tomography process, a spatial distribution of x-ray photon intensities merging from a body under examination are translated into analog electric signals which are processed in a manner that enables reconstructing the x-ray image and displaying it as a visible image. Background information on the process is given in an article by Gordon, et al., "Image Reconstruction from Projections", Scientific American, October, 1975, Vol. 233, No. 4.
In computed axial tomography systems, detectors must detect x-ray photons efficiently and with a high degree of spatial resolution. In some systems, the x-ray source is pulsed and the pulsed repetition rate can be limited by the recovery time of the x-ray detectors. It is desirable to use x-ray detectors which have fast recovery time, high sensitivity, and fine spatial resolution. In multicell detectors, it is also important for each cell to have identical and stable detecting characteristics.
In some tomography systems, the x-ray bean is fan-shaped and diverges as it exits from the examination subject whereupon the beam falls on the array of detector cells such that photon intensities over the leading front of the beam can be detected and resolved spatially. As the x-ray source and detector oribts around the examination subject jointly, the x-ray intensities across the diverging beam projected from the source are detected by the individual detector cells and corresponding analog electric signals are produced. The individual detector cells are arranged in a stack or array so that the x-ray photon distribution across the beam at any instant are detected simultaneously. The signals correspond with x-ray absorption along each ray path at the instant of detection. Additional sets of signals are obtained for the several angular positions of the orbiting detector and x-ray source. The discrete analog signals are converted to digital signals and are processed in a computer which is controlled by a suitable algorithm to produce signals representative of the absorption by each small volume element in the examination subject through which the fan-shaped x-ray beam passes.
To get good spatial resolution, it is desirable to have the electrode plates, which comprise each cell, spaced closely and uniformly over the entire length of the detector. A detector which has advanced achievement of these results is disclosed in U.S. Pat. No. 4,119,853, entitled "Multicell X-ray Detector" to Shelley, et al., and is assigned to the assignee of the present application. The detector in the cited patent comprises a plurality of adjacent, but slightly spaced apart, electrode plates standing edgewise so as to define gas filled gaps between them in which ionization events, that is, the production of the electron-ion pairs due to photon interaction with the gas, may take place. Improved spacing and dimensional tolerances are achieved by securing the electrode plates in a unitary electrode assembly. The structure of the cited detector comprises a pair of flat metal bars which are curved in their planes and constitute a segment of a circle to form the upper and lower frame for the assembly. The bars are substantially congruent with each other in spaced apart parallel planes. There are spacers between the ends of the bars to maintain their spacing. Similarly, curved insulating members which support the electrode plates are bonded to the facing sides of the respective bars. The insulating members have circumferentially spaced radially extending grooves machined in them. Grooves in opposite members lie on the same radii. A viscous resin coating, such as an epoxy resin, is spread over the entire grooved face of each member. The upper and lower edges of an array and electrode plates are inserted in corresponding grooves in the respective insulating members. An epoxy interface is formed between the upper and lower edges of the electrode plates and the walls of the slots. The bonding method also result in bridges of epoxy being developed between the adjacent electrode plates as shown in FIGS. 1 and 1a. The epoxy resin is cured to produce a solid bond of each plate. Alternate electrode plates are connected together and then connected to a common potential source and are called the bias electrodes. The signal electrodes, constituting the electrode plates intervening between every other bias electrode plate, have their own individual connections leading to a data signal acquisition system, which is exterior of the detector. The unitary electrode assembly is disposed within a pressure vessel or chamber which has an internal channel that is curved complimentarily with the electrode assembly. The front wall of the chamber has a relatively thin section, constituting an x-ray transmissive window. A cover is secured to the chamber to close the open top of the channel, and a sealing gasket is disposed between the cover and the chamber. Means are provided for pressurizing the interior of the chamber with a high atomic weight gas, such as xenon, at about 25 atmospheres to adapt the detector for use with x-rays adding photon energy in the range of up to 120 kilo electron volts.
A common problem associated with the detector of the prior art results from high frequency mechanical vibration and is known as microphonics. The electrode plates are made of extremely thin metal and must operate in close proximity with a relatively large potential difference between them. Mechanical vibrations can be transmitted through the gas chamber to the electrode assembly and to the electrode plates. Such vibrations may significantly vary the capacitance between electrodes, particularly where the plates have differing rigidity, and can introduce microphonic current changes, which cause errors in the x-ray intensity measurements. These spurious microphonic currents are in the picoampere range, but are comparable to the x-ray induced signal and have been erroneously measured as signals in prior art detectors even though no x-ray photons were present.
Another common problem associated with the detector of the prior art results from low frequency distortions due to thermal variations of the electrode assembly over the operating range of the detector. The thermal expansion can create relative distortion between the electrode plates to significantly vary the spacing between the electrodes and introduce microphonic currents and inconsistent responses which may cause errors in the x-ray intensity measurements.
A particular problem is presented by the prior art method of bonding the electrode plates to the members. It was previously believed that the bridges of epoxy which were formed by capillary action between the plates were beneficial to stabilize the plates. It was recently discovered that not only is the excessive epoxy not beneficial, it significantly contributes to microphonics by making some plates more rigid than others, and to thermal instability due to the different coefficients of expansion of the plates and the epoxy which distorts the cells spacing at different operating temperatures of the detector.